1. Field of the Invention
The present invention relates to a display device having a plurality of current driven self-emitting elements such as, for example, an organic electroluminescence element, and wherein display is realized by controlling the electric current to the self-emitting elements.
2. Description of the Related Art
Because electroluminescence (hereinafter simply referred to as “EL”) display devices wherein an EL element which is a self-emitting element is used as an emissive element in each pixel have advantages, such as that the device is self-emitting, that the thickness can be reduced, and that the power consumption is small, they have attracted much attention as alternative to conventional display devices such as liquid crystal display (LCD) and CRT (cathode ray tube) display devices, and, consequently, a great deal of effort is focused on development of EL display devices.
In particular, active matrix EL display devices in which one or more switching elements such as a thin film transistor (hereinafter simply referred to as “TFT”) for individually controlling the EL element are provided in each pixel and an EL element for each pixel is controlled are highly expected to become available as high resolution display devices.
FIG. 4 shows a circuit structure in each pixel in an active matrix EL display device having m rows and n columns. In an EL display device, a plurality of gate lines GL extend along the row direction on a substrate and a plurality of data lines DL and driving power supply lines VL extend along the column direction on the substrate. Each pixel has an organic EL element 50, a switching TFT (first TFT) 10, an EL element driving TFT (second TFT) 21, and a storage capacitor Cs.
The first TFT 10 is connected to a gate line GL and a data line DL and is switched on when a gate signal (selection signal) is received at its gate. When the first TFT is switched on, a data signal supplied on the data line DL is stored in the storage capacitor Cs which is connected between the first TFT 10 and the second TFT 21. A voltage corresponding to the data signal supplied through the first TFT 10 is supplied to the gate electrode of the second TFT 21 and the second TFT 21 supplies a current corresponding to the supplied voltage value from the power supply line VL to the organic EL element 50. In the organic EL element, holes injected from an anode and electrons injected from a cathode recombine within an emissive layer to excite emissive molecules and light is emitted as the excited molecules return from the excited state to the ground state. The emission brightness of the organic EL element 50 is approximately proportional to the current supplied to the organic EL element 50. Thus, by controlling the current flowing through the organic EL element 50 corresponding to the data signal in each pixel as described above, light of a brightness corresponding to the data signal is emitted from the organic EL element, and a desired image is displayed by the overall display device.
As described, an organic EL element 50 is a current driven element and requires that a sufficient amount of current be supplied from the power supply line VL through the second TFT to the organic EL element 50. In addition, a plurality of organic EL elements 50 are connected to a power supply line VL. Therefore, in order to supply a sufficient amount of current to these organic EL elements 50, the power supply line VL must have sufficient current supply capability. In order to connect the power supply line VL and a second TFT 21, a contact is employed. That is, a drain region of the second TFT and the power supply line VL are provided at different positions in the thickness direction and a contact formed extending in the thickness direction is used to connect these components.
As the contact, a conductive material is layered within a contact hole along the inner surface of the contact hole. Because of this, the length of the current path from the power supply line VL through the contact becomes long. Because the size of the contact is usually smaller than the width of the power supply line VL, a portion of the power supply line VL remains around the contact, the portion having a very small area. Therefore, there had been a problem in that formation of the contact results in an increase in the resistance in the power supply line VL.
Although, on the other hand, a power supply line VL having a sufficient width is employed, no such problem is created, a power supply line having a minimum width is demanded in order to achieve higher resolution and higher aperture ratio in each pixel.